Nonstoichiometric charge defect induced relaxor antiferroelectric ordering in La modified Bi0.5 (Na0.80K0.20)0.5TiO3 relaxor ferroelectric

Abstract

The effect of A-site nonstoichiometric in La3+ modified Bi0.5(Na0.80K0.20)0.5TiO3/BNKT solid solution on crystal structure and electrical properties have been investigated. Evolution of crystal structure from rhombohedral (R3c) to tetragonal (P4bm) symmetry with reduction of the lattice distortion has been observed. The Raman spectra analysis is well supported to the crystal structure study from the XRD patterns analysis. The presence of charge defects due to non-stoichiometric composition and the reduction of oxygen vacancies have been confirmed by the x-ray photoelectron spectra (XPS) of the respective elements. The FESEM (field emission scanning electron microscopy) micrographs clearly show the reduction of grain size with the incorporation of La3+ in A-site of BNKT. The multiple dielectric anomalies near depolarization temperature (Td) and temperature at maximum dielectric value (Tm) in the temperature dependent dielectric curves ascribe the diffuse phase transition with relaxor ferroelectric nature. The dielectric anomalies at Td and Tm become more pronounce with the increase in addition of La3+ into BNKT lattice. The presence of charge defects with A-sites complexity leads to interrupt the long range polar ferroelectric domains and, forms the polar nanoregions (PNRs). The competition between ferroelectric R3c symmetry and non-polar P4bm symmetry with the addition of La3+ into the BNKT lattice induces an intermediate modulated phase, which exhibits the antiferroelectric in nature having pinched ferroelectric hysteresis loop.